def prost(x):
    k=0
    for d in range(1,x+1):
        if x%d==0:
            k+=1
    if  k==2:
     return True
    else:
     return False
maxx=0
num=int(input())
if num>=1000 and num<=9999:
    n1=str(num)
    for chifra in range(len(n1)):
        if prost(int(n1[chifra])):    
            maxx=max(maxx,int(n1[chifra]))
    for chislo in range(len(n1)-1):
        if prost (int(n1[chislo]+n1[chislo+1]))==True:
            maxx=max(maxx,int(n1[chislo]+n1[chislo+1]))
    for chislo in range(len(n1)-2):
        if prost (int(n1[chislo]+n1[chislo+1]+n1[chislo+2]))==True:
            maxx=max(maxx,int(n1[chislo]+n1[chislo+1]+n1[chislo+2]))
    for chislo in range(len(n1)-3):
        if prost (int(n1[chislo]+n1[chislo+1]+n1[chislo+2]+n1[chislo+3]))==True:
           maxx=max(maxx,int(n1[chislo]+n1[chislo+1]+n1[chislo+2]+n1[chislo+3]))
    print(maxx)
else:
    print(0)
